Nonlinear wavelength conversion, producing tunable laser wavelengths, is very useful in a wide range of applications. It is always desirable to have high efficiency for nonlinear wavelength conversion. The power conversion efficiency of the nonlinear optical process strongly depends on the length of the crystal, the pump power, and the nonlinear coupling coefficient. To maximize the effective nonlinear coefficient in a nonlinear wavelength conversion process, L. E. Myers et al. has disclosed a kind of quasi-phase-matched nonlinear crystal, called periodically poled lithium niobate, in Quasi-phase-matched 1.064-μm Pumped Optical Parametric Oscillator in Bulk Periodically Poled LiNbO3 in Opt. Lett. Vol. 20 pp. 52–54 (1995). When the pump power is low and the nonlinear coefficient is fixed, the length of a nonlinear optical crystal often limits power efficiency in nonlinear wavelength conversion. In the low-efficiency regime, power efficiency in a nonlinear optical waveguide is proportional to the square of the crystal length, whereas that in a bulk nonlinear optical crystal is linearly proportional to the crystal length due to diffraction. In the high-efficiency regime, power gain for parametric amplification can grow exponentially along the crystal length. Most nonlinear optical crystals are expensive and not easy to grow, and therefore typical nonlinear crystals have a size varying from a few millimeters to a few centimeters. Although it is possible to access a longer effective gain length by traversing optical waves several times in a nonlinear optical material via internal or external reflections, the phase matching condition for nonlinear wavelength conversion is often destroyed upon reflection. To increase the gain length in a nonlinear crystal of a finite size, T. H. Jeys disclosed a multiple-pass optical parametric amplifier in Opt. Lett. Vol. 21, pp. 1229–1231 (1996). However, Jeys's device has no phase correction between passes and is only applicable to broadband optical parametrical generation from vacuum noises. A double-pass second-harmonic generation with mechanical phase correction was disclosed by G. Imeshev et al. in Phase Correction in Double-pass Quasi-phase-matched Second-harmonic Generation with a Wedged Crystal, Opt. Lett. Vol. 23, pp. 165–167 (1998). However, mechanical phase correction is unstable and slow.
It is therefore attempted by the applicants to deal with the above difficulties encountered in a multiple-pass nonlinear wavelength conversion process. It is known in the field that the much faster and more stable electro-optic effect can alter the refractive index of and the phase of an optical wave in a second-order nonlinear optical material. The present invention employs an electro-optic phase compensator properly integrated to a multiple-pass nonlinear wavelength converter to achieve high-efficiency nonlinear wavelength conversion. On the other hand, signal modulation is often necessary for sensitive detection and information transmission in various laser applications. It is desirable to have a high-efficiency nonlinear frequency converter with a built-in convenient modulator. The present invention has the additional advantage of using the high-speed electro-optic phase compensator to function as a high-speed amplitude modulator to the nonlinear mixing waves.